The content  will be uploaded to whatever site collection contains the site. In order for content to go into the new content database, you will need to move a site collection to the new content db. The way I advise to move site collections is using backup and restore:

technet.microsoft.com/en-us/library/cc706871.aspx

Taking a content database offline just means that no NEW site collections will be added it when a new site collection is created but it is not recommended to leave a database offline. Use the site limits option to control the number of site collections within a content database. This article describes the issues with leaving a database offline on a SharePoint server with an SSP:

support.microsoft.com/default.aspx/kb/2000856